This invention relates to navigable vessels and one of its principal objects is to provide a vessel with improved performance, particularly a vessel that creates less turbulence, has less frictional resistance, and performs better in turbulent water. The result of such improvements is increased fuel economy and/or speed, smoother operation and less structural demands on the vessel.
A vessel moving through water experiences frictional resistance at the wetted surface below the water line. As the speed of the vessel increases the turbulence created by the hull moving through the water increases rapidly until frictional forces become the practical barrier to higher speed. The energy required to propel the vessel increases correspondingly. Improving speed and efficiency are recognized as the primary goals and activities in the naval arts and decreasing frictional resistance is seen as the key to these goals.
Numerous vessel designs have been proposed for reducing resistance. Planing hulls are widely used in moderate size and smaller vessels. The planing surfaces on the hull cause the vessel to rise in the water as speed increases, thus decreasing the wetted surface area and thereby decreasing the frictional resistance and drag. This decrease can be substantial. Nevertheless, a major part of the wetted surface remains, together with its associated frictional resistance and drag.
To reduce the wetted surface to a greater extent hydrofoil have been employed. Hydrofoils, like airfoils (e.g. wings) in the aeronautical arts, are streamlined bodies which create a useful reaction ("lifting force") from a fluid stream moving relative to them. In practice hydrofoils are given a different curvature (camber) at the opposed surfaces. The resulting unbalanced profile is designed to create an efficient lifting force in the water at the selected angle of attack of the hydrofoil, i.e. the angle between the chord (straight line connecting the leading and trailing edge) of the hydrofoil and the direction of movement of the vessel. The hydrofoils are secured to the hull of the vessel and usually extend transversely amidships, at and/or below the bottom of the hull.
Hydrofoils are capable of lifting the vessel almost completely from the water, thus reducing friction and drag to that imparted by the remaining relatively minor amount of wetted surface (principally portions of the propulsion system, and the relatively hydrodynamically efficient rudder and hydrofoils). However, the formidable structural and other design problems involved in lifting an entire vessel onto hydrofoils and continuing to propel it limits their use to smaller vessels. These vessels have additional serious shortcomings. They have poor stability and are difficult to handle. They have limited service speed. Hydrofoils are highly vulnerable to floating debris. Moreover, hydrofoils, as designed and positioned, can only impart a lifting action and they serve no appreciable function of aligning flow or otherwise decreasing the friction of the water on the vessel hull when a portion of the hull is under water at slower speeds. Indeed, the foils likely add to turbulence and drag when the hull is in the water.
An extremely thin layer of water immediately next to the surface of the vessel below the water line (wetted surface) is termed the boundary layer. In this layer most of the shear forces of frictional resistance take place. Another proposal for reducing friction on a vessel is the introduction into the boundary layer of agents which will lower the friction in this layer by lowering the viscosity or by some other mechanism. Various agents have been proposed, including microscopic air or other gas bubbles, certain polymers, polysaccharides, and petroleum products. This technique appears to have potential but there are serious drawbacks and limitations in the means which have been proposed for generating, in the case of microbubbles, and the means proposed for introducing and distributing all such agents.
U.S. Pat. Nos. 661,303 and 2,378,822 each disclose an attachment for the bow of a vessel which is mounted over the apex and extends aft for a distance. Gas is discharged at holes or ports along the aft margins. At least in the case of U.S. Pat. No. 2,378,822 the forwardly facing portion of the attachment is streamlined to match to some extent the bow which it covers. This will reduce to some degree turbulence created by the attachment as compared to a totally bluff body. However, the added resistance and drag due to this structure will be substantial because elements of the attachment necessarily extend to the outside of the bow, thus creating turbulence, particularly at the necked in stern portion and at the discontinuities created by the sternward edges. Additionally as the location of the bubble release is aft of the bow apex, there is no coverage at the area which is typically the most important with regard to creation of turbulence. Nor are these attachments capable of effectively spreading bubbles appreciably outward of the boundary layer at the bow for the purpose of reducing viscosity in the sublayers. In other disclosures nozzles or perforated diffuser pipes are positioned below the waterline in advance of the bow to emit bubbles, using air supplied by connecting pipes from the vessel. Full coverage with bubbles of the field in advance of the bow is possible in this fashion. However, these systems are constituted of bluff bodies (having blunt shapes that create a rapid increase of pressure gradient downstream) which themselves cause appreciable turbulence in the water passing the bow, thus they detract significantly from any friction reduction on the hull by the emitted bubbles.
Microbubbles of diameters less than 60 microns have been found to be the most effective in reducing friction. Their generation has been achieved by diffusing air under pressure through a microporous plate, as described in the papers of Madavan, Deutsh and Merkle published in the Journal of Fluid Mechanics (1985), vol. 156, pp. 237-245, titled "Measurement of local skin friction in a microbubble-modified turbulent boundary layer" and in Phys. Fluids 27 (2), February 1984, entitled "Reduction of turbulent friction by microbubbles" and in the references cited in these papers. However the energy required for this is so great as to even exceed the energy saved through the friction reduction by the microbubbles generated.
Bow rudders have been proposed as a supplement to or replacement of a rear rudder. Conventional rudders, though, even if streamlined, create an undesirable amount of turbulence and drag in a turning mode, particularly in a bow placement. This is because the turbulence created will move along the wetted surface of the bow thus further increasing resistance and drag on the wetted surface. Additionally, pivoting at the bow the relatively large mass of the rudder decreases the stability of the vessel.
Japanese patent 55-136692 describes a pivoting front section of the bow to overcome the stability problem. However, this solution introduces its own problems, i.e. the difficulty and complexity of pivotably mounting, supporting and operating a rather massive structure. French patent 956,241 discloses fins or wings pivotably mounted on a bulb at the bow and extending aft from the pivot. The pivot axis are vertical so that pivoting of the fins will divert flow to create a turning force at the bow. Due to placement of these fins adjacent to and outwardly from the bow, they will inevitably create considerable turbulence along the hull, even if they are streamlined.
Another mode of decreasing friction is to heat the wetted surfaces of the vessel. U.S. Pat. No. 3,452,701 discloses a rudder used as a heat exchanger for cooling engine coolant by circulating the coolant through a heat exchanger coil inside the rudder to the stern of the rudder post. However, no friction reduction effect is contemplated and, if achieved would be incidental.